THS4522IPWR datasheet(36/71 Pages) TI2

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V(MAX)

V )

2R (2R

A )

2R R A

2R (2

A )

R A (4

A )

2R (2

A )

R A (4

THS4521, THS4522, THS4524

SBOS458H – DECEMBER 2008 – REVISED JUNE 2015

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Device Functional Modes (continued)

8.4.1.3 Resistor Design Equations for the Single-Ended to Differential Configuration of the FDA

The design equations for setting the resistors around an FDA to convert from a single-ended input signal to

differential output can be approached from several directions. Here, several critical assumptions are made to

simplify the results:

•

The feedback resistors are selected first and set equal on the two sides.

•

The dc and ac impedances from the summing junctions back to the signal source and ground (or a bias

voltage on the non-signal input side) are set equal to retain feedback divider balance on each side of the

FDA.

Both of these assumptions are typical for delivering the best dynamic range through the FDA signal path.

After the feedback resistor values are chosen, the aim is to solve for the RT (a termination resistor to ground on

the signal input side), RG1 (the input gain resistor for the signal path), and RG2 (the matching gain resistor on the

nonsignal input side); see Figure 74 and Figure 75. The same resistor solutions can be applied to either ac- or

dc-coupled paths. Adding blocking capacitors in the input-signal chain is a simple option. Adding these blocking

capacitors after the RT element (as shown in Figure 74) has the advantage of removing any dc currents in the

feedback path from the output VOCM to ground.

Earlier approaches to the solutions for RT and RG1 (when the input must be matched to a source impedance, RS)

follow an iterative approach. This complexity arises from the active input impedance at the RG1 input. When the

FDA is used to convert a single-ended signal to differential, the common-mode input voltage at the FDA inputs

must move with the input signal to generate the inverted output signal as a current in the RG2 element. A more

recent solution is shown as Equation 1, where a quadratic in RT can be solved for an exact value. This quadratic

emerges from the simultaneous solution for a matched input impedance and target gain. The only inputs required

are:

1. The selected RF value.

2. The target voltage gain (Av) from the input of RT to the differential output voltage.

3. The desired input impedance at the junction of RT and RG1 to match RS.

Solving this quadratic for RT starts the solution sequence, as shown in Equation 1:

(1)

Being a quadratic, there are limits to the range of solutions. Specifically, after RF and RS are chosen, there is

physically a maximum gain beyond which Equation 1 starts to solve for negative RT values (if input matching is a

requirement). With RF selected, use Equation 2 to verify that the maximum gain is greater than the desired gain.

(2)

If the achievable AV(MAX) is less than desired, increase the RF value. After RT is derived from Equation 1, the RG1

element is given by Equation 3:

(3)

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